The invention relates to systems for protecting the passengers of an airplane against the effects of cabin depressurization at high altitude by providing them with the oxygen they need to survive.
In most present systems, the principle used is as shown in FIG. 1. The airplane carries a source of oxygen (an oxygen cylinder, a chemical generator known as a “candle”, or an on-board generator for generating air that is pressurized and highly enriched in oxygen). The source feeds one or more general distribution pipes. Each seat for a passenger is provided with at least one mouth-and-nose mask 10 connected to the general pipe 12 via a feed path that includes a breathe-in non-return check valve 14, a flexible economizer bag 16, a coupling tube 18 having a constriction 20 for limiting flow rate, and a cock (not shown) which opens when the passenger pulls the mask in order to press it against the face. The mask also has a breathe-out valve 22 and an additional breathe-in valve 24 that is rated so as to present a small amount of resistance. If the rate at which oxygen is admitted from the bag is less than the instantaneous breathe-in demand from the wearer of the mask, valve 24 makes it possible to inhale an additional quantity of air from the outside.
The flexible economizer bag enables the contant flow coming from the source to adapt to the breathing cycle of the wearer: the economizer bag 16 stores the oxygen supplied during the breathe-out stage of the cycle. Its inflated volume generally lies in the range 500 milliliters (ml) to 1000 ml. The amount of oxygen stored in this way is available during the following inhalation and is additional to the quantity of oxygen that continues to be supplied through the constriction 20.
The continuous flow rate supplied by the oxygen source is conventionally expressed in terms of volume per minute, where volume is reduced to normal temperature and pressure conditions when dry (NTPD).
Current Federal Aviation Regulation (FAR) 25 1443 C makes it necessary for the control unit which sets the flow rate delivered to the mask by adjusting the pressure upstream from the constrictions feeding the masks to operate in such a manner that the total NTPD flow rate of oxygen supplied to each passenger varies:                from 3.8 liters per minute (l/min) to 0.75 l/min when altitude varies from 40,000 feet (ft) to 18,500 ft (i.e. approximately 12,200 meters (m) to 5600 m); and        from 0.75 l/min to 0 when altitude varies from 18,500 ft to 10,000 ft (5600 m to 3050 m).        
That type of operation leads to a relationship between flow rate and altitude presenting a discontinuity at 18,500 ft. This discontinuity can be seen in FIG. 2 which shows a typical variation curve, plotting minima as a function of flight altitude.
In a conventional device using an economizer bag, only 0.3 l/min to 0.6 l/min NTPD of oxygen is actually consumed by the metabolic requirements of the wearer, depending on whether the wearer is calm or stressed. The major portion of the oxygen supplied is thus dumped to the ambient medium together with the gas breathed out. Of the oxygen that is administered, the fraction that is genuinely needed thus lies in the range about 15% at high altitude to less than 30% at lower altitude.